The main flaw I see is asking for a refund of a non-refundable ticket

Also, I would check what fare bases are eligable for the 2 for 1; often the Ba ones are only on the full fare type tickets

The J r/t fare to PHX is €5380 plus taxes; the fare which you , I think, are looking at is an I fare at €1759 plus taxes or R fare at €2059 plus taxes

Before going any further I would contact the airline and verify what the cheapest eligable fare is. I suspect that you are going to be up for, even if they allow the change ( which I doubt ) for the difference between the €2400 and ( €5380 plus 2 lots of taxes )

Dave